Laboratory leaching tests of bottom and fly ash samples from an oil shale fueled pulverized firing (PF) and a circulated fluidized bed (CFB) boiler in 1:10 (wt/vol) suspensions of deionized water were performed. The ash samples were characterized for BETN2 specific surface area, mesopore distribution and contents of elements by ICP-MS. Water extracts were analyzed for pH, conductivity and the contents of 22 elements by ICP-MS. Several toxic metals, e.g. Tl, Mo, Sr, Cr and Pb, exhibited higher mobilization rates in fly ash samples from the PF boiler after two sequential extraction cycles compared to fly ashes from the CFB boiler. Higher mobility of trace elements in fly ash samples from the PF boiler may be explained by higher temperature in the furnace chamber of the PF boiler compared to the CFB boiler and surface characteristics of the ashes. According to the results the release of some trace elements, such as V, Cr, Ni, Cu, Zn, Mo, Cd, Ba, Tl and Pb from oil shale ash to aqueous environment is a long-term process. (C) 2014 Elsevier B.V. All rights reserved.